Semicrystalline polymer/graphene oxide composite film and method for fabricating the same

ABSTRACT

The present invention discloses a semicrystalline polymer/graphene oxide composite film, comprising: a first semicrystalline-typed polymer, distributed in structural space of the composite film and having a porous structure; and graphene oxide, having a layered structure and distributed in the composite film wherein gas passage exist between adjacent layered structures, the first semicrystalline-typed polymer existing between part of adjacent layered structures forms into a second semicrystalline-typed polymer by further heat treatment after the first semicrystalline-typed polymer and graphene oxide are blended uniformly to be distributed in the composite film so as to fill and seal a portion of the porous structure to block gas from flowing to extend path length(s) of gas passage; wherein graphene oxide existing between the first semicrystalline-typed polymers induces formation of the second semicrystalline-typed polymer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to a composite film andmethod thereof, and more particularly to a semicrystallinepolymer/graphene oxide composite film and method thereof.

2. Description of the Prior Art

Accompanying with the advance of technologies, new mobile or portableinformation electronic products keep continuously emerging and a flatpanel display thereon has to be applicable to the flexible displaytechnology and flexible electronic devices in addition to being light,thin, compact, etc.

A barrier film is a barrier layer to block gas and moisture from flowingso as to keep gas and moisture on one side of the film. Therefore, sucha barrier film can be applied in food wrapping, packaging, solar cells,flexible displays, etc. and plays an important role in electronic andpackaging industries. Since organic and metallic substances usually usedin electronic industries are sensitive to oxygen and moisture, passageof moisture and oxygen may cause metal in electronic components tooxidize so as to affect performance of a display and shorten lifetime ofa display. Therefore, an electronic product should be properly packagedfor suitable protection.

A flexible display becomes the current trend for a new era of displayresearch where the substrate for a flexible display is a flexibleplastic substrate instead of glass. However, there are manymanufacturing problems to be conquered in order to provide a usablereliable flexible plastic substrate. Among these, a flexible plasticsubstrate having high transparency and high size stability is urgentlyneeded. The size stability and limitation of operational temperature ofa flexible plastic substrate is a big limiting factor. On the otherhand, gas barrier processing for a flexible plastic substrate is oneother key factor to determine the lifetime of a flexible display and tomaintain display quality.

Besides, it is reported that polyvinyl alcohol being a water solublepolymer to be high hydrophilic, biocompatible, nontoxic, and capable offilm forming (H. M. Kim, J. K. Lee, H. S. Lee, Transparent and high gasbarrier films based on poly(vinyl alcohol)/graphene oxide composites,Thin Solid Films, In Press, Corrected Proof.). A polyvinyl alcohol filmhas the characteristics of high transparency (transmittance >90%) andbeing a good gas barrier. In addition, it is a dense film with highcrystallinity and has good adhesiveness, oil and solvent resistance.Regarding its mechanical properties, a polyvinyl alcohol film has goodtoughness and large tensile strength (44.1˜63.7) to have good tearresistance among various plastic films so as to be extensively used infood packaging. However, the polyvinyl alcohol film fulfills therequirements as a gas barrier for food packaging but requires furtherimprovement for electronic industries, such as functioning as asubstrate of a flexible display.

Furthermore, oxygen transport and free volume in cold-crystallized andmelt-crystallized poly(ethylene naphthalate) (PEN) is reported by Y. S.Hu, R. Y. F. Liu, L. Q. Zhang, M. Rogunova, etc. (Macromolecules 2002,35, 7326-7337) where crystallization enhances gas barrier property ofPEN even for small gas molecules. A Study of the Gas Barrier Propertiesof Highly Oriented Polyethylene is reported by P. S. HOLDEN, G. A. J.ORCHARD, I. M. WARD, etc. (Journal of Polymer Science: Polymer PhysicsEdition, Vol. 23, 709-731 (1985)) where crystallization enhances gasbarrier property of polyethylene film even for small gas molecules.

A graphene-made film has high gas barrier property as well but it lackstransparency and flexibility when its film thickness becomes thicker.Therefore, a graphene film cannot be standalone as a gas barrier and maybe suitable to combine with other polymers such as polyvinyl alcohol tobe used as a composite material to maintain gas barrier propertytogether with transparency.

On the other hand, a gas barrier film with high transparency formed byblending polyvinyl alcohol water soluble polymer with clay is reportedbut the gas barrier film becomes brittle to be unsuitable in flexibleapplications because there are a lot of clay needed to have the gasblockage effect. Besides, a composite film formed by mixing polymer withlayered graphene oxide is not reported.

Therefore, it is urgently needed to have a good gas barrier film withhigh transparency suitable to be coated on a substrate for a flexibledisplay.

SUMMARY OF THE INVENTION

In light of the above background, in order to fulfill the requirementsof industries, one object of the present invention is to provide asemicrystalline polymer/graphene oxide composite film and methodthereof, to effectively form a good gas barrier film with hightransparency.

Another object of the present invention is to provide a semicrystallinepolymer/graphene oxide composite film formed by blending graphene oxidewith a polymeric solution and using solution casting and dry phaseinversion methods so as to extend the length of gas passage to enhancethe gas barrier performance.

One other object of the present invention is to provide asemicrystalline polymer/graphene oxide composite film to achieve thepurpose of enhancing gas barrier effect by adding an optimum amount ofgraphene oxide and utilizing the crystalline characteristic of a polymerto block gas from flowing through or prolong the length of gas passagefrom one side of the film to the other side by processing thesemicrystalline polymer with heat to cause recrystallization ofpolymeric molecule among gaps of the layered graphene oxide molecules.

Accordingly, the present invention discloses a semicrystallinepolymer/graphene oxide composite film, comprising: a firstsemicrystalline-typed polymer, distributed in structural space of thecomposite film and having a porous structure; and graphene oxide, havinga layered structure and distributed in the composite film wherein gaspassage exist between adjacent layered structures, the firstsemicrystalline-typed polymer existing between part of adjacent layeredstructures forms into a second semicrystalline-typed polymer by furtherheat treatment after the first semicrystalline-typed polymer andgraphene oxide are blended uniformly to be distributed in the compositefilm so as to fill and seal a portion of the porous structure to blockgas from flowing to extend path length(s) of gas passage; whereingraphene oxide existing between the first semicrystalline-typed polymersinduces formation of the second semicrystalline-typed polymer. Thepolymer is selected from the group consisting of the following orcombination thereof: polyvinyl alcohol (PVA), ethylene-vinyl alcohol(EVOH) copolymer, polyethylene terephthalate (PET), polypropylene (PP),olyvinylidene chloride, polyetheretherketon (PEEK), and ethylene vinylacetate copolymer.

In one embodiment, the polymer is polyvinyl alcohol and thesemicrystalline polymer/graphene oxide composite film is semicrystallinepolyvinyl alcohol/graphene oxide composite film having a film thicknessof 5˜100 μm. Preferably, the semicrystalline polymer/graphene oxidecomposite film is semicrystalline polyvinyl alcohol/graphene oxidecomposite film having a film thickness of 10 μm. The semicrystallinepolymer/graphene oxide composite film is a semicrystalline polyvinylalcohol/graphene oxide composite film and a weight ratio ofsemicrystalline-typed polyvinyl alcohol to graphene oxide of thecomposite film is 1000:1˜50:1. Preferably, the weight ratio ofsemicrystalline polyvinyl alcohol to graphene oxide of the compositefilm is 1000:1.

Accordingly, the present invention discloses a method for fabricating asemicrystalline polymer/graphene oxide composite film, comprising:providing a polymer/graphene oxide casting solution by dissolving apolymer in a first solvent to obtain a mixture solution, adding agraphene oxide solution into the mixture solution, and stirring untiluniform wherein the graphene oxide solution comprises a second solventand graphene oxide dispersed in the second solvent; performing a filmforming procedure by coating the polymer/graphene oxide casting solutionon a substrate to form a first film on the substrate wherein thegraphene oxide in the first film has a layered structure and distributedin the polymer; performing a crystallization procedure by drying thefirst film with heat to form the first film having a firstsemicrystalline-typed polymer wherein the first semicrystalline-typedpolymer has a porous structure and the first semicrystalline-typedpolymer and graphene oxide are distributed uniformly in the first filmwith respect to each other so as to have the first semicrystalline-typedpolymer exist between graphene oxide and graphene oxide also existbetween the first semicrystalline-typed polymers; performing arecrystallization procedure by having the composite film having thefirst semicrystalline-typed polymer undergo heat treatment to induce thefirst semicrystalline-typed polymer polyvinyl alcohol between adjacentlayers of graphene oxide to form a second semicrystalline-typed polymerso as to obtain a composite film having the first semicrystalline-typedpolymer and the second semicrystalline-typed polymer wherein the secondsemicrystalline-typed polymer fills and seals the porous structure ofthe first semicrystalline-typed polymer to block gas from flowingthrough and to increase a length that gas passes from one side of thecomposite film to the other side of the composite film.

According to the method of the present invention, the graphene oxidesolution is prepared by dissolving sodium nitrate in concentratedsulfuric acid by heating and stirring to obtain a sodiumnitrate/sulfuric acid solution; adding graphene oxide into the sodiumnitrate/sulfuric acid solution and stirring until uniform to obtain afirst mixture; lowering the temperature of the first mixture by placingin an ice bath; slowly adding potassium permanganate (KMnO₄) after thefirst mixture is stabilized; slowly adding deionized water; setting thesolution to obtain graphene oxide precipitation; performing vacuumdrying to obtain graphene oxide flakes; and dispersing graphene oxide indeionized water by supersonic oscillation to ensure graphene oxidecompletely being dispersed in deionized water and being exfoliated witha single layered structure so as to obtain the graphene oxide solution.

The first solvent is deionized water. The mixture solution is apolyvinyl alcohol solution and comprises polyvinyl alcohol and deionizedwater with a weight ratio (polyvinyl alcohol/deionized water) of1:100˜20:100. Preferably, the mixture solution is a polyvinyl alcoholsolution and comprises polyvinyl alcohol and deionized water with aweight ratio (polyvinyl alcohol/deionized water) of 5:95. The secondsolvent is deionized water. The graphene oxide solution comprisesgraphene oxide and deionized water with a weight ratio (grapheneoxide/deionized water) of 1:50000˜1:1000. Preferably, the graphene oxidesolution comprises graphene oxide and deionized water with a weightratio (graphene oxide/deionized water) of 1:10000.

According to the method of the present invention, the polymer ispolyvinyl alcohol and the step of performing a crystallization procedureby drying the first film with heat comprises: drying the first film at80˜100° C. for 1˜3 hr; and processing the first film by vacuum dryingfor 18˜24 hr at room temperature to have polyvinyl alcohol form into afirst semicrystalline-typed polyvinyl alcohol. The heat treatment in therecrystallization procedure is controlled within a temperature range of80˜140° C. After the recrystallization procedure, the method furthercomprises: performing a quenching procedure by placing the compositefilm in a temperature range of 3˜15° C. The polymer is polyvinyl alcoholand the semicrystalline polymer/graphene oxide composite film issemicrystalline polyvinyl alcohol/graphene oxide composite film having afilm thickness of 5˜100 μm. Preferably, the semicrystallinepolymer/graphene oxide composite film is semicrystalline polyvinylalcohol/graphene oxide composite film having a film thickness of 10 μm.The step of performing a film forming procedure by coating thepolymer/graphene oxide casting solution on a substrate is performed byusing a doctor blade to form a wet film with a thickness of 100˜1000 μm.The step of performing a film forming procedure by coating thepolymer/graphene oxide casting solution on a substrate is performed byusing a doctor blade to form a wet first film with a thickness of 300μm. The polymer is polyvinyl alcohol, the semicrystallinepolymer/graphene oxide composite film is semicrystalline polyvinylalcohol/graphene oxide composite film and a weight ratio of thesemicrystalline polyvinyl alcohol to graphene oxide of the compositefilm is 1000:1˜50:1. Preferably, the weight ratio of the semicrystallinepolyvinyl alcohol to graphene oxide of the composite film is 1000:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional schematic diagram illustrating astructure of a semicrystalline polymer/graphene oxide composite filmaccording to one embodiment of the present invention;

FIG. 2 shows a cross-sectional schematic diagram illustrating astructure of a semicrystalline polyvinyl alcohol/graphene oxidecomposite film according to one embodiment of the present invention;

FIG. 3 shows a schematic diagram illustrating forming a semicrystallinepolymer/graphene oxide composite film by a doctor blade according to oneembodiment of the present invention;

FIG. 4A shows a schematic diagram illustrating the relationship betweenthe addition quantity of graphene oxide and oxygen transmission rate(OTR) in the semicrystalline polymer/graphene oxide composite filmaccording to one embodiment of the present invention;

FIG. 4B shows a schematic diagram illustrating the relationship betweencrystallinity and recrystallization time at 100° C. for pure PVA and PVAadded with 0.1 wt % graphene oxide in the semicrystallinepolymer/graphene oxide composite film according to one embodiment of thepresent invention;

FIG. 5 shows a schematic diagram illustrating X-ray diffraction spectraof the semicrystalline polymer/graphene oxide composite film accordingto one embodiment of the present invention;

FIG. 6 shows a schematic diagram illustrating surfaces and crosssections of polyvinyl alcohol/polyethylene terephthalate film andpolyvinyl alcohol/graphene oxide composite film viewed by SEM accordingto one embodiment of the present invention;

FIG. 7 shows a schematic diagram illustrating DSC (differential scanningcalorimeter) results of polyvinyl alcohol according to one embodiment ofthe present invention;

FIG. 8 shows a schematic diagram illustrating the relationship betweenheat treatment time and OTR of the semicrystalline polymer/grapheneoxide composite film; and

FIG. 9 shows a schematic diagram illustrating the relationship betweenthe addition quantity of graphene oxide and transmittance according toone embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

What is probed into the invention is a semicrystalline polymer/grapheneoxide composite film. Detail descriptions of the structure and elementswill be provided in the following in order to make the inventionthoroughly understood. Obviously, the application of the invention isnot confined to specific details familiar to those who are skilled inthe art. On the other hand, the common structures, elements, andprocesses that are known to everyone are not described in details toavoid unnecessary limits of the invention. Some preferred embodiments ofthe present invention will now be described in greater detail in thefollowing. However, it should be recognized that the present inventioncan be practiced in a wide range of other embodiments besides thoseexplicitly described, that is, this invention can also be appliedextensively to other embodiments, and the scope of the present inventionis expressly not limited except as specified in the accompanying claims.

In one embodiment of the present invention, a semicrystallinepolymer/graphene oxide composite film is provided. The composite film isformed by a polymer and graphene oxide. Since graphene oxide has alayered structure, graphene oxide can be blended with a semicrystallinepolymer and the layered graphene oxide can induce the semicrystallinepolymer for further recrystallization by heat treatment. Because layeredgraphene oxide exists or distributes among the skeleton (structuralspace) of the semicrystalline polymer, new semicrystalline-typed polymeris formed between the layered structure of graphene oxide so as togenerate a composite film containing semicrystalline polymer andgraphene oxide with a specific structure. The polymer is selected fromthe group consisting of the following or combination thereof: polyvinylalcohol (PVA), ethylene-vinyl alcohol (EVOH) copolymer, polyethyleneterephthalate (PET), polypropylene (PP), olyvinylidene chloride,polyetheretherketone (PEEK), and ethylene vinyl acetate copolymer.

FIG. 1 shows a cross-sectional schematic diagram illustrating astructure of a semicrystalline polymer/graphene oxide composite filmaccording to one embodiment of the present invention. Thesemicrystalline polymer/graphene oxide composite film 100 comprises alayered compound 101 (graphene oxide), a first semicrystalline-typedpolymer 102 and a second semicrystalline-typed polymer 103. In thesemicrystalline polymer/graphene oxide composite film 100, the layeredcompound 101 (graphene oxide) exists in empty space of the skeleton orstructure of the composite film 100 and the first semicrystalline-typedpolymer 102 has a porous structure. The polymer is in a crystallinestate. The layered compound 101 forms into a plurality of layeredstructures or has a multiple-layer form and is distributed or dispersedbetween the first semicrystalline-typed polymers 102. The firstsemicrystalline-typed polymer 102 between some adjacent layeredstructures can transform into the second semicrystalline-typed polymer103 to block gas from flowing through. In this way, the secondsemicrystalline-typed polymer 103 fills and seals the porous structureand prolongs the length of gas passage 106. That is, gas to pass throughthe composite film requires going through a longer path.

The method for fabricating the above mentioned semicrystallinepolymer/graphene oxide composite film comprises the following steps:

Step 1: preparing a polymeric solution by dissolving a polymer in asolvent to obtain a polymeric casting solution with 5˜20 wt % ofpolymer;

Step 2: having the polymeric casting solution undergo pressurizedfiltration to remove impurities in the polymeric casting solution;

Step 3: adding a solution dispersed with a layered compound to obtain apolymer/layered compound casting solution 401 with 1˜10 wt % ofpolymer/layered compound;

Step 4: setting the polymer/layered compound casting solution 401 for aday to remove bubbles in the solution for subsequent solution casting;

Step 5: coating the polymer/layered compound casting solution 401 on aflexible plastic substrate by a doctor blade to form a first film with athickness of 100˜300 μm;

Step 6: drying the first film by placing the first film in an 50˜120° C.oven for 0.5˜3 hr to remove the solvent in the first film;

Step 7: further processing the first film by vacuum drying for 20˜30 hrat room temperature to completely remove the solvent in the first filmso as to form a composite film having the first semicrystalline-typedpolymer 102 with a film thickness of 5˜100 μm, preferably 10 μm;Step 8: placing the composite film in an oven for a predetermined periodof time for heat treatment so as to form a composite film 100 having thesecond semicrystalline-typed polymer 103; andStep 9: quenching the composite film 100 having the secondsemicrystalline-typed polymer 103 by placing the heat-treated compositefilm 100 from the oven into a refrigerator at about 3˜15° C. to preventpolymeric chains of crystalline zones and non-crystalline zones frombeing disturbed.

In another embodiment of the present invention, a semicrystallinepolyvinyl alcohol/graphene oxide composite film 300 is provided. Thecomposite film 300 comprises graphene oxide 301, a firstsemicrystalline-typed polyvinyl alcohol 302 and a secondsemicrystalline-typed polyvinyl alcohol 303. In the composite film 300,the first semicrystalline-typed polyvinyl alcohol 302 forms into aporous structure in empty space of the skeleton or structure of thecomposite film 300. The first semicrystalline-typed polyvinyl alcohol isin a crystalline state. The graphene oxide 301 forms into a plurality oflayered structures or has a multiple-layer form and is distributed ordispersed between the first semicrystalline-typed polyvinyl alcohol 302.The first semicrystalline-typed polyvinyl alcohol 302 between someadjacent layered structures can transform into the secondsemicrystalline-typed polyvinyl alcohol 303 to block gas from flowingthrough. In this way, the second semicrystalline-typed polyvinyl alcohol303 fills and seals the porous structure and prolongs the length of gaspassage 306. That is, gas to pass through the composite film requiresgoing through a longer path. As shown in FIG. 2, the method forfabricating the above mentioned semicrystalline polymer/graphene oxidecomposite film 300 comprises the following steps:

Step 1: adding sodium nitrate and concentrated sulfuric acid in a beakerand heating to 80° C. and stirring until sodium nitrate is completelydissolved in sulfuric acid;

Step 2: adding graphene oxide into sodium nitrate/sulfuric acid solutionand stirring for 2 hr until uniform;

Step 3: placing in an ice bath for 20 min to lower the temperature;

Step 4: slowly adding potassium permanganate (KMnO₄) after the mixturesolution is stabilized;

Step 5: slowly adding deionized water after 2 hr of reaction and stayingin the ice bath because a large amount of heat will be generated;

Step 6: slowly adding hydrogen peroxide to reduce manganese ion;

Step 7: adding deionized water and setting the solution to a standstillfor a day to obtain precipitation (graphene oxide);

Step 8: rinsing precipitation (graphene oxide) by HCl and deionizedwater repeatedly; and

Step 9: using a dialysis bag to wash graphene oxide until pH=7.0.

In this embodiment, after step 9, the following steps are proceeded:

Step 10: performing vacuum drying to obtain graphene oxide flakes;

Step 11: dispersing graphene oxide in water by supersonic oscillationfor more than 1 hr to ensure graphene oxide completely being dispersedin water and being exfoliated with a single layered structure

Step 12: preparing a polyvinyl alcohol casting solution by dissolvingpolyvinyl alcohol in deionized water, stirring in an oil bath for 1 hr,and then stirring under room temperature for 30 min so as to obtain a 10wt % polyvinyl alcohol casting solution;

Step 13: having the polyvinyl alcohol casting solution undergopressurized filtration to remove impurities in the polyvinyl alcoholcasting solution;

Step 14: adding the solution having different concentration of grapheneoxide and stirring until uniform so as to obtain a polyvinylalcohol/graphene oxide casting solution 401 with 5 wt % of polyvinylalcohol/graphene oxide; and

Step 15: setting the polyvinyl alcohol/graphene oxide casting solution401 for a day to remove bubbles in the solution for subsequent solutioncasting.

In this embodiment, after step 15, the following steps are proceeded:

Step 16: coating the casting solution 401 on a PET flexible plasticsubstrate 403 by a doctor blade, as shown in FIG. 3, to form a firstfilm 404 with a thickness of 300 μm;

Step 17: drying the first film by placing the first film 404 in an 90°C. oven for 1 hr to remove the solvent in the first film 404;

Step 18: further processing the first film by vacuum drying for 24 hr atroom temperature to completely remove the solvent in the first film 404so as to form a composite film having the first semicrystalline-typedpolyvinyl alcohol with a film thickness of 5˜100 μm, preferably 10 μm;Step 19: placing the composite film in an oven for a predeterminedperiod of time for heat treatment so as to form a composite film 300having the second semicrystalline-typed polyvinyl alcohol; andStep 20: quenching the composite film 300 having the secondsemicrystalline-typed polyvinyl alcohol by placing the heat-treatedcomposite film 300 from the oven into a refrigerator at about 3˜15° C.,preferably 5° C., to prevent polymeric chains of crystalline zones andnon-crystalline zones from being disturbed.

According to one embodiment of the present invention, a semicrystallinepolyvinyl alcohol/graphene oxide composite film formed by polyvinylalcohol and graphene oxide is provided. Since the crystallinity (Xc)affects gas permeability, the crystallinity of the composite film formedby adding the different quantity of graphene oxide into polyvinylalcohol is tested. From table 1, it is found that adding the differentquantity of graphene oxide into polyvinyl alcohol does not significantlyaffect the crystallinity of the composite film, that is, the additionamount of graphene oxide in the range of the present invention does notsignificantly affect crystallization behavior of polyvinyl alcohol.Therefore, the effect of the crystallinity to gas permeability under theaddition amount of graphene oxide in the range of the present inventioncan be ignored.

TABLE 1 DSC result for PVA/ GO composite films with different GOconcentration GO concentration (wt %) Crystallinity (%) 0 32.4 0.1 32.460.25 32.02 0.5 31.91 1 31.77

According to another embodiment of the present invention, asemicrystalline polyvinyl alcohol/graphene oxide composite film byadding graphene oxide into polyvinyl alcohol is provided. FIG. 4A showsa schematic diagram illustrating the relationship between the additionquantity of graphene oxide and oxygen transmission rate (OTR) in thesemicrystalline polymer/graphene oxide composite film according to oneembodiment of the present invention. From the figure, it is found thatOTR decreases and then slowly increases when the addition quantityincreases. When the addition quantity of graphene oxide is 0.1 wt %, thelowest OTR (0.025 cc/m² day) is obtained. When the addition quantity ofgraphene oxide is 0.1 wt %, OTR of the polyvinyl alcohol/graphene oxidefilm is lower about 80%, compared with the polyvinyl alcohol/PET film.The DSC results (data) show that the crystallinity of polyvinyl alcoholis not affected significantly by adding or doping graphene oxide.Therefore, it is confirmed that graphene oxide in polyvinyl alcoholpossesses its gas barrier characteristic.

Please refer to FIG. 5. FIG. 5 shows a schematic diagram illustratingX-ray diffraction spectra of the semicrystalline polymer/graphene oxidecomposite film according to one embodiment of the present invention. AnX-ray diffraction analyzer and a field-emission scanning electronmicroscope are used to analyze cases that graphene oxide is dispersed(scattered) in polyvinyl alcohol. Since there is no graphene oxidecharacteristic peak at about 10°, it means that graphene oxide isscattered or dispersed in polyvinyl alcohol to be exfoliated orintercalated for the addition quantity of graphene oxide (<1 wt %).Therefore, no graphene oxide characteristic peak indicates that grapheneoxide has high gas barrier characteristic within this range, as shown inFIG. 4A.

FIG. 6 shows a schematic diagram illustrating surfaces and crosssections of polyvinyl alcohol/polyethylene terephthalate film andsemicrystalline polyvinyl alcohol/graphene oxide composite film viewedby SEM according to one embodiment of the present invention where (a),(c), (e), and (g) show surfaces containing 0.1 wt %, 0.25 wt %, 0.5 wt%, and 1.0 wt %, respectively; and (b), (d), (f), and (h) show crosssections containing 0.1 wt %, 0.25 wt %, 0.5 wt %, and 1.0 wt %,respectively. From the figure, the surface having graphene oxideaggregates for 1.0 wt % graphene oxide causes the gas barriercharacteristic to be lowered, as shown in FIG. 4A. In addition, from thecross sections, it is found that polyvinyl alcohol and thesemicrystalline polyvinyl alcohol/graphene oxide composite film bothform a dense thin film on the polyethylene terephthalate substrate, witha thickness about 10 μm. It shows that gas barrier performance is notclosely related to the thickness of polyvinyl alcohol.

According to another embodiment of the present invention, the effect ofconstant temperature recrystallization to the gas barrier characteristicof the semicrystalline polyvinyl alcohol/graphene oxide composite filmis discovered. The differential scanning calorimeter (DSC) spectra ofpolyvinyl alcohol is shown in FIG. 7. The crystallization temperaturerange of the semicrystalline polyvinyl alcohol is about 80˜140° C. Thus,in this embodiment, the heat treatment for constant temperaturerecrystallization to the composite film is performed within the abovetemperature range so as to form the second semicrystalline-typedpolyvinyl alcohol in the composite film for enhancing gas barrierperformance. Under oxygen transmission rate tests, after graphene oxideis added, compared to the composite without the secondsemicrystalline-typed polyvinyl alcohol but only with the firstsemicrystalline-typed polyvinyl alcohol, the composite film with thesecond semicrystalline-typed polyvinyl alcohol has lower oxygentransmission rate. Therefore, the optimum addition quantity of grapheneoxide is 0.1 wt %, as shown in FIG. 4A. Therefore, at 80˜140° C.,preferably 100° C., the composite film is processed with heatedtreatment for constant temperature recrystallization for differentduration and tested with oxygen transmission rate. From FIG. 4B, it isfound that the composite film processed for 1˜6 hr, preferably 6 hr ofrecrystallization can become a semicrystalline polyvinylalcohol/graphene oxide composite film having a secondsemicrystalline-typed polyvinyl alcohol having crystallinity increasedto 46.56% from 32.46%. As shown in FIG. 4B, it is found thatcrystallinity is increased by 100° C. of recrystallization processingand 0.1 wt % graphene oxide addition. Therefore, it shows that grapheneoxide functions as a nucleating agent to induce formation of the secondsemicrystalline-typed polyvinyl alcohol and the newly formedsemicrystalline-typed polyvinyl alcohol surrounds graphene oxide in thecomposite film.

According to this embodiment, in the oxygen transmission rate test shownin FIG. 8, the semicrystalline polyvinyl alcohol/graphene oxidecomposite film after heat treatment has significantly lower OTR thanbefore heat treatment. It shows that the second semicrystalline-typedpolyvinyl alcohol can effectively increase the length of the gas passagepath so as to decrease the gas transmission rate to achieve therequirement of gas barrier. When the recrystallization durationincreases, crystallinity is increased to effectively reduce OTR.

According to the embodiments of the present invention, the relationshipbetween the addition quantity of graphene oxide and transmittance forthe semicrystalline polyvinyl alcohol/graphene oxide composite film isshown in FIG. 9. The composite films with different quantities ofgraphene oxide have different transmittance. Although PVA istransparent, the transmittance gradually decreases when the additionquantity of graphene oxide increases. Generally, the transmittance of85% is a limit for a flexible display and thus transmittance for acomposite film having graphene oxide less than 0.25 wt % has nosignificant difference with pure PVA while transmittance for a compositefilm having graphene oxide more than 0.25 wt % becomes seriously lower.Therefore, when the addition quantity of graphene oxide is less than0.25 wt %, the composite film is applicable to applications related toflexible displays.

According to the embodiments of the present invention, graphene oxidehaving a single-layered structure uniformly dispersed in PVA can befabricated so as to obtain the composite film. Under the condition ofthe addition quantity of graphene oxide being 0.1 wt %, the oxygentransmission rate of the composite film has a 80% decrease compared to apure PVA film and the oxygen transmission rate of the composite filmbeing further processed by constant temperature crystallization tocomprise the second semicrystalline-typed polyvinyl alcohol has a morethan 94% decrease compared to a pure PVA film.

Obviously many modifications and variations are possible in light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims the present invention can be practiced otherwisethan as specifically described herein. Although specific embodimentshave been illustrated and described herein, it is obvious to thoseskilled in the art that many modifications of the present invention maybe made without departing from what is intended to be limited solely bythe appended claims.

What is claimed is:
 1. A semicrystalline polymer/graphene oxidecomposite film, consisting of a first semicrystalline polymer which ispolyvinyl alcohol, distributed in structural space of the composite filmand having a porous structure, a second semicrystalline polymer formedfrom a part of the first semicrystalline polymer, and a graphene oxide,wherein the graphene oxide has a layered structure and is distributed inthe composite film, wherein a pathway of gas passage exists betweenadjacent layered structures of the graphene oxide, wherein a weightratio of polyvinyl alcohol to graphene oxide in the composite film is200:1 to 100:1, wherein the second semicrystalline polymer was formedfrom the part of the first semicrystalline polymer that existed betweena portion of the adjacent layered structures of the graphene oxide byapplying a heat treatment after the first semicrystalline polymer andgraphene oxide were blended uniformly and distributed in the compositefilm, wherein the second semicrystalline polymer fills and seals aportion of the porous structure to block gas from flowing and extendsthe path length(s) of the pathway of gas passage, wherein thecrystallinity of the first semicrystalline polymer is less than 32%,wherein the crystallinity of the second semicrystalline polymer isgreater than 32%, and wherein said semicrystalline polymer/grapheneoxide composite film is characterized by an X-ray powder diffractionpattern comprising peaks at 20.1+0.2, 22.5±0.2 and 29.5±0.2 2-thetadegree.
 2. The composite film according to claim 1, having a filmthickness of 20 to 100 μm.
 3. The composite film according to claim 1,wherein the weight ratio of polyvinyl alcohol to the graphene oxide inthe composite film is 200:1.
 4. The composite film according to claim 1,wherein the weight ratio of polyvinyl alcohol to graphene oxide in thecomposite film is 100:1.